Most conventional speech coders apply the same coding method regardless of the local character of the speech segment to be encoded. It is, however, recognized that enhanced quality can be achieved if the coding method is changed, or adapted, according to the local character of the speech. Such adaptive methods are commonly based on some form of classification of a given speech segment, which classification is used to select one of several coding modes (multi-mode coding). Such techniques are especially useful when there is background noise which, in order to obtain a natural sounding reproduction thereof, requires coding approaches that differ from the coding technique generally applied to the speech signal itself.
One disadvantage associated with the aforementioned classification schemes is that they are somewhat rigid; giving rise to the danger of mis-classifying a given speech segment and, as a result, selecting an improper coding mode for that segment. The improper coding mode typically results in severe degradation in the resulting coded speech signal. The classification approach thus disadvantageously limits the performance of the speech coder.
A well-known technique in multi-mode coding is to perform a closed-loop mode decision where the coder tries all modes and decides on the best according to some criterion. This alleviates the mis-classification problem to some extent, but it is a problem to find a good criterion for such a scheme. It is, as is also the case for aforementioned classification schemes, necessary to transmit information (i.e., send overhead bits from the transmitter's encoder through the communication channel to the receiver's decoder) describing which mode is chosen. This restricts the number of coding modes in practice.
It is therefore desirable to permit a speech coding (encoding or decoding) procedure to be changed or adapted based on the local character of the speech without the severe degradations associated with the aforementioned conventional classification approaches and without requiring transmission of overhead bits to describe the selected adaptation.
According to the present invention, a speech coding (encoding or decoding) procedure can be adapted without rigid classifications and the attendant risk of severe degradation of the coded speech signal, and without requiring transmission of overhead bits to describe the selected adaptation. The adaptation is based on parameters already existing in the coder (encoder or decoder) and therefore no extra information has to be transmitted to describe the adaptation. This makes possible a completely soft adaptation scheme where an infinite number of modifications of the coding (encoding or decoding) method is possible. Furthermore, the adaptation is based on the coder's characterization of the signal and the adaptation is made according to how well the basic coding approach works for a certain speech segment.